Thermal Disk Emission Research Articles

FIELD AND GLOBULAR CLUSTER LOW-MASS X-RAY BINARIES IN NGC 4278

We report a detailed spectral analysis of the population of low-mass X-ray binaries (LMXBs) detected in the elliptical galaxy NGC~4278 with Chandra. Seven luminous sources were studied individually, four in globular clusters (GCs), and three in the stellar field. The range of (0.3-8 keV) $L_X$ for these sources suggests that they may be black hole binaries (BHBs). Comparison of our results with simulations allows us to discriminate between disk and power-law dominated emission, pointing to spectral/luminosity variability, reminiscent of Galactic BHBs. The BH masses derived from a comparison of our spectral results with the $L_X \sim T^4_{in}$ relation of Galactic BHBs are in the 5-15 $M_{\odot}$ range, as observed in the Milky Way. The analysis of joint spectra of sources selected in three luminosity ranges suggests that while the high luminosity sources have prominent thermal disk emission components, power-law components are likely to be important in the mid and low-luminosity spectra. Comparing low-luminosity average spectra, we find a relatively larger $N_H$ in the GC spectrum; we speculate that this may point to either a metallicity effect, or to intrinsic physical differences between field and GC accreting binaries. Analysis of average sample properties uncover a previously unreported $L_X - R_G$ correlation (where $R_G$ is the galactocentric radius) in the GC-LMXB sample, implying richer LMXB populations in more central GCs. No such trend is seen in the field LMXB sample. We can exclude that the GC $L_X - R_G$ correlation is the by-product of a luminosity effect, and suggest that it may be related to the presence of more compact GCs at smaller galactocentric radii, fostering more efficient binary formation.

Accretion discs in blazars

The characteristic properties of blazars (rapid variability, strong polarization and high brightness) are widely attributed to a powerful relativistic jet oriented close to our line of sight. Despite the spectral energy distributions being strongly jet-dominated, a ‘big blue bump’ has been recently detected in the sources known as flat spectrum radio quasars (FSRQs). These new data provide a unique opportunity to observationally test coupled jet–disc accretion models in these extreme sources. In particular, as energy and angular momentum can be extracted by a jet magnetically coupled to the accretion disc, the thermal disc emission spectrum may be modified from that predicted by the standard model for disc accretion. We compare the theoretically predicted jet-modified accretion disc spectra against the new observations of the ‘big blue bump’ in FSRQs. We find mass accretion rates that are higher, typically by a factor of 2, than predicted by standard accretion disc theory. Furthermore, our results predict that the high-redshift blazars PKS 0836+710, PKS 2149−307, B2 0743+25 and PKS 0537−286 may be predominantly powered by a low- or moderate-spin (a≲ 0.6) black hole with high-mass accretion rates ⁠, while 3C 273 harbours a rapidly spinning black hole (a= 0.97) with ⁠. We also find that the black hole masses in these high-redshift sources must be ≳5 × 109 M⊙.

A Search for Synchrotron X‐Ray Emission in Radio Quasars

This paper presents XMM-Newton and Chandra X-ray spectroscopy of ten flat-spectrum radio quasars (FSRQ) which are candidates to have an X-ray spectrum dominated by jet synchrotron emission. In all these FSRQ, which are less strongly relativistically beamed than blazars, a considerable contribution from a power-law component similar to that present in radio-quiet quasars is required to explain the observed X-ray fluxes and X-ray spectral slopes. And as in radio-quiet quasars, their relatively high optical/UV fluxes can be accounted for by a significant contribution from thermal accretion disk emission. The lack of success in finding radio quasars with synchrotron X-rays is attributed to the adopted selection criteria, which were based on the multiwavelength flux ratios of BL Lacertae (BL Lac) objects. A refined selection technique, which additionally involves radio imaging, is proposed to search for these important candidates for the Gamma Ray Large Area Space Telescope (GLAST). On the other hand, the discovered FSRQ with their strong accretion disk signatures are expected to be important probes for studies of the poorly known accretion disk - jet connection.

XMM-Newtonobservations of 4 luminous radio-quiet AGN, and the soft X-ray excess problem

Context. The nature and origin of the soft X-ray excess in radio-quiet AGN is still an open issue. The interpretation in terms of thermal disc emission has been challanged by the discovery of the constancy of the effective temperature despite the wide range of Black Hole masses of the observed sources. Alternative models are reflection from ionized matter and absorption in a relativistically smeared wind. Aims. We analyzed XMM-Newton observations of four luminous radio-quiet AGN with the aim of characterising their main properties and in particular the soft excess. Methods. Different spectral models for the soft excess were tried: thermal disc emission, Comptonization, ionized reflection, relativistically smeared winds. Results. Comptonization of thermal emission and smeared winds provide the best fits, but the other models also provide acceptable fits. All models, however, return parameters very similar from source to source, despite the large differences in luminosities, Black Hole masses and Eddington ratios. Moreover, the smeared wind model requires very large smearing velocities. The UV to X-ray fluxes ratios are very different, but do not correlate with any other parameter. Conclusions. No fully satisfactory explanation for the soft X-ray excess is found. More informative data, such as observations in a broader energy band, are needed to make further progress.

Multiwavelength XMM-Newton observations of the Laor et al. sample of PG quasars

We present XMM–Newton/EPIC spectra for the Laor et al. sample of Palomar Green (PG) quasars. We find that a power law provides a reasonable fit to the 2–5 keV region of the spectra. Excess soft X-ray emission below 2 keV is present for all objects, with the exception of those known to contain a warm absorber. However, a single power law is a poor fit to the 0.3–10.0 keV spectrum and instead we find that a simple model, consisting of a broken power law (plus an iron line), provides a reasonable fit in most cases. The equivalent width of the emission line is constrained in just 12 objects but with low (<2σ) significance in most cases. For the sources whose spectra are well fitted by the broken-power-law model, we find that various optical and X-ray line and continuum parameters are well correlated; in particular, the power-law photon index is well correlated with the FWHM of the Hβ line and the photon indices of the low- and high-energy components of the broken power law are well correlated with each other. These results suggest that the 0.3–10 keV X-ray emission shares a common (presumably non-thermal) origin, as opposed to suggestions that the soft excess is directly produced by thermal disc emission or via an additional spectral component. We present XMM–Newton Optical Monitor (OM) data, which we combine with the X-ray spectra so as to produce broad-band spectral energy distributions (SEDs), free from uncertainties due to long-term variability in non-simultaneous data. Fitting these optical–UV spectra with a Comptonized disc model indicates that the soft X-ray excess is independent of the accretion disc, confirming our interpretation of the tight correlation between the hard and soft X-ray spectra.

The XMM-Newtonview of GRS 1915+105

(abridged) Two XMM-Newton observations of the black-hole binary GRS1915+105 were triggered in 2004, during a long plateau state of the source. (...) While the light curves show just small amplitude variations (a few percent) at timescales longer than a few seconds, a QPO is seen at about 0.6 Hz (...). The pn spectrum is well fitted without invoking thermal disk emission, on the base of four main components: a primary one (...), absorbed by cold matter with abundances different than those of standard ISM; reprocessing from an ionized disk; emission and absorption lines; and a soft X-ray excess around 1 keV. However, the latter is not confirmed by the RGS spectra, whose difference from the EPIC-pn ones actually lacks of a fully satisfactory explanation. If real, the soft X-ray excess may be due to reflection from an optically thin, photoionized disk wind; in this case it may yield a way to disentangle intrinsic from interstellar absorption.

Microwave Emission from Spinning Dust in Circumstellar Disks

In the high-density environments of circumstellar disks dust grains are expected to grow to large sizes by coagulation. Somewhat unexpectedly, recent near-IR observations of PAH features from disks around Herbig Ae/Be stars demonstrate that a substantial amount of dust mass in the surface layers of these disks (up to several tens of percent of the local carbon content) can be locked up in small nanometer-sized particles. We investigate the possibility of detecting the electric dipole emission produced by these nanoparticles (sizes ~1 nm) as they spin at thermal rates (tens of GHz) in cold gas. We show that such emission peaks in the microwave range and dominates over the thermal disk emission at ν 50 GHz typically by a factor of several if 5% of the total carbon abundance is locked up in nanoparticles. We test the sensitivity of this prediction to various stellar and disk parameters and show that if the potential contamination of the spinning dust component by the free-free and/or synchrotron emission can be removed, then the best chances of detecting this emission would be in disks with small opacities, having SEDs with steep submillimeter slopes (which minimize thermal disk emission at GHz frequencies). Detection of the spinning dust emission would provide important evidence for the existence, properties, and origin of the population of small dust particles in protoplanetary disks, with possible ramifications for planet formation.

Photometric Accretion Signatures Near the Substellar Boundary

Multi-epoch imaging of the Orion equatorial region by the Sloan Digital Sky Survey has revealed that significant variability in the blue continuum persists into the late-M spectral types, indicating that magnetospheric accretion processes occur below the substellar boundary in the Orion OB1 association. We investigate the strength of the accretion-related continuum veiling by comparing the reddening-invariant colors of the most highly variable stars against those of main sequence M dwarfs and evolutionary models. A gradual decrease in the g band veiling is seen for the cooler and less massive members, as expected for a declining accretion rate with decreasing mass. We also see evidence that the temperature of the accretion shock decreases in the very low mass regime, reflecting a reduction in the energy flux carried by the accretion columns. We find that the near-IR excess attributed to circumstellar disk thermal emission drops rapidly for spectral types later than M4. This is likely due to the decrease in color contrast between the disk and the cooler stellar photosphere. Since accretion, which requires a substantial stellar magnetic field and the presence of a circumstellar disk, is inferred for masses down to 0.05 Msol we surmise that brown dwarfs and low mass stars share a common mode of formation.

Probing the Ionizing Continuum of Narrow‐Line Seyfert 1 Galaxies. I. Observational Results

We present optical spectra and emission-line ratios of 12 narrow-line Seyfert 1 (NLS1) galaxies that we observed to study the ionizing EUV continuum. A common feature in the EUV continuum of active galactic nuclei is the big blue bump (BBB), generally associated with thermal accretion disk emission. While Galactic absorption prevents direct access to the EUV range, it can be mapped by measuring the strength of a variety of forbidden optical emission lines that respond to different EUV continuum regions. We find that narrow emission line ratios involving [O II] λ3727, Hβ, [O III] λ5007, [O I] λ6300, Hα, [N II] λ6583, and [S II] λλ6716, 6731 indicate no significant difference between NLS1 galaxies and broad-line Seyfert 1 (BLS1) galaxies, which suggests that the spectral energy distributions of their ionizing EUV-soft X-ray continua are similar. The relative strength of important forbidden high-ionization lines like [Ne V] λ3426 compared to He II λ4686 and the relative strength of [Fe X] λ6374 appear to show the same range as in BLS1 galaxies. However, a trend of weaker F([O I] λ6300)/F(Hα) emission-line ratios is indicated for NLS1 galaxies compared to BLS1 galaxies. To recover the broad emission line profiles, we used Gaussian components. This approach indicates that the broad Hβ profile can be well described with a broad component (FWHM 3275 ± 800 km s-1) and an intermediate broad component (FWHM 1200 ± 300 km s-1). The width of the broad component is in the typical range of normal BLS1 galaxies. The emission-line flux that is associated with the broad component in these NLS1 galaxies amounts to at least 60% of the total flux. Thus, it dominates the total line flux, similar to BLS1 galaxies.

On the observed disc temperature of accreting intermediate mass black holes

Some ultraluminous X-ray sources in nearby galaxies show soft components resembling thermal disc emission. Calculations based on blackbody emission then indicate that the accreting black holes at the centres of these discs have masses of 100s to 1000s Msun. Establishing the existence of such intermediate mass black holes is of considerable importance so the assumptions and approximations lying behind blackbody spectral fits must be examined. We study here the basic assumption that the thermal emission is well-characterised by a multi-temperature blackbody spectrum. Since the opacity in the surface layers of a disc decreases at high energies the emergent spectrum is hardened. We compute the observed spectra from discs around a non-spinning 1000 Msun black hole and fit them over the XMM-Newton pn band with multi-colour disc models. The typical overall spectra hardening factor usually adopted for discs around stellar mass black hole (assuming an inclination of 60 deg and including relativistic effects for a non-spinning black hole) is found to be appropriate.

Constraining X‐Ray Binary Jet Models via Reflection

Although thermal disk emission is suppressed or absent in the hard state of X-ray binaries, the presence of a cold, thin disk can be inferred from signatures of reprocessing in the ~2-50 keV band. The strength of this signature is dependent on the source spectrum and flux impinging on the disk surface and is thus very sensitive to the system geometry. The general weakness of this feature in the hard state has been attributed to either a truncation of the thin disk, large ionization, or beaming of the corona region away from the disk with β ~ 0.3. This latter velocity is comparable to jet nozzle velocities, so we explore whether a jet can account for the observed reflection fractions. It has been suggested that jets may contribute to the high-energy spectra of X-ray binaries, via either synchrotron from around (100-1000)rg along the jet axis or from inverse Compton (synchrotron self-Compton and/or external Compton) from near the base. Here we calculate the reflection fraction from jet models wherein either synchrotron or Compton processes dominate the emission. Using as a guide a data set for GX 339-4, for which the reflection fraction previously has been estimated as ~10%, we study the results for a jet model. We find that the synchrotron case gives less than 2% reflection, while a model with predominantly synchrotron self-Compton in the base gives ~10%-18%. This shows for the first time that an X-ray binary jet is capable of significant reflection fractions and that extreme values of the reflection may be used as a way of discerning the dominant contributions to the X-ray spectrum.

The remarkable rapid X-ray, ultraviolet, optical and infrared variability in the black hole XTE J1118+480

The transient black-hole binary XTE J1118+480 exhibited dramatic rapid variability at all wavelengths which were suitably observed during its 2000 April–July outburst. We examine time-resolved X-ray, ultraviolet, optical and infrared data spanning the plateau phase of the outburst. We find that both X-ray and infrared bands show large amplitude variability. The ultraviolet and optical variability is more subdued, but clearly correlated with that seen in the X-rays. The ultraviolet, at least, appears to be dominated by the continuum, although the lines are also variable. Using the X-ray variations as a reference point, we find that the ultraviolet (UV) variability at long wavelengths occurs later than that at short wavelengths. Uncertainty in the Hubble Space Telescope timing prohibits a determination of the absolute lag with respect to the X-rays, however. The transfer function is clearly not a delta-function, exhibiting significant repeatable structure. For the main signal we can rule out an origin in reprocessing on the companion star – the lack of variation in the lags is not consistent with this, given a relatively high orbital inclination. Weak reprocessing from the disc and/or companion star may be present, but is not required, and another component must dominate the variability. This could be variable synchrotron emission correlated with X-ray variability, consistent with our earlier interpretation of the infrared (IR) flux as due to synchrotron emission rather than thermal disc emission. In fact, the broad-band energy distribution of the variability from IR to X-rays is consistent with expectations of optically thin synchrotron emission. We also follow the evolution of the low-frequency quasi-periodic oscillation in X-rays, UV, and optical. Its properties at all wavelengths are similar, indicating a common origin.

The Remarkably Featureless High-Resolution X-Ray Spectrum of Markarian 478

An observation of Mrk 478 using the Chandra Low Energy Transmission Grating Spectrometer is presented. The source exhibited 30%–40% flux variations on timescales of ∼104 s together with a slow decline in the spectral softness over the full 80 ks observation. The 0.15–3.0 keV spectrum is well fitted by a single power law with photon index Γ = 2.91 ± 0.03. Combined with high-energy data from BeppoSAX, the spectrum from 0.15 to 10 keV is well fitted as the sum of two power laws with Γ1 = 3.03 ± 0.04, which dominates below 2 keV, and Γ2 = 1.4 ± 0.2, which dominates above 2 keV (quoting 90% confidence uncertainties). No significant emission or absorption features are detected in the high-resolution spectrum, supporting our previous findings using the Extreme Ultraviolet Explorer but contradicting the earlier claims of emission lines by Hwang & Bowyer. There is no evidence of a warm absorber as found in the high-resolution spectra of many Seyfert 1 galaxies, including others classified as narrow-line Seyfert 1's such as Mrk 478. We suggest that the X-ray continuum may result from Comptonization of disk thermal emission in a hot corona through a range of optical depths.